At present the chargers for handsets and digital cameras mostly adopt a switch power supply circuit. They are designed for a lower power supply and can be classified as follow: a first type in which the circuit on the primary side of a transformer adopts RCC separated elements to form self actuation control or a PWM controller to form external actuation control; a second type in which the secondary side of a transformer adopts a dual operational charge circuit to achieve constant current and a voltage base circuit to achieve constant voltage, or adopts a voltage base circuit to achieve constant voltage and a transistor to achieve constant current; and a third type which adopts an ASIC (application specific integrated circuit) design which has a dedicated integrated circuit on the primary side of a transformer to control output at constant voltage and constant current without an optical coupler and a constant voltage/current controller on the secondary side of the transformer.
The first type is simpler, but has notable disadvantages, such as greater element dispersion, lower efficiency, no short circuit function, lower production yield, thus the acceptance is dwindling now. The third type that adopts the ASIC design is simpler, but it requires a special technology available only to a small number of manufacturers. Moreover, it does not provide a vigorous voltage/current control circuit, hence system harmonic wave and noise performances are less desirable, and the cost also is higher, thus it still cannot fully meet customers' requirements. The second type is most widely adopted. Its system performance can meet the constant voltage/current requirement of most high performance chargers. Environmental temperature variation also does not have significant impact on the system performance. However, it also has its share of problems, such as system cost is higher and circuit board area is greater.
To overcome the aforesaid problems, a higher performance solution is needed to provide a higher efficiency, improved short circuit characteristics, and lower system output harmonic wave and system cost.